Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS. 19 



species in being grafted together. As in reciprocal 

 crosses, the facility of effecting an union is often very 

 far from equal, so it sometimes is in grafting ; the 

 common gooseberry, for instance, cannot be grafted on 

 the currant, whereas the currant will take, though with 

 difficulty, on the gooseberry. 



We have seen that the sterility of hybrids, which 

 have their reproductive organs in an imperfect con- 

 dition, is a different case from the difficulty of uniting 

 two pure species, which have their reproductive organs 

 perfect ; yet these two distinct classes of cases run to 

 a large extent parallel. Something analogous occurs 

 in grafting ; for Thouin found that three species of 

 Eobinia, which seeded freely on their own roots, and 

 which could be grafted with no great difficulty on a 

 fourth species, when thus grafted were rendered barren. 

 On the other hand, certain species of Sorbus, when 

 grafted on other species yielded twice as much fruit as 

 when on their own roots. We are reminded by this 

 latter fact of the extraordinary cases of Hippeastrum, 

 Passiflora, &c., which seed much more freely when 

 fertilised with the pollen of a distinct species, than 

 when fertilised with pollen from the same plant. 



We thus see, that, although there is a clear and great 



difference between the mere adhesion of grafted stocks, 



and the union of the male and female elements in the 



act of reproduction, yet that there is a rude degree of 



parallelism in the results of grafting and of crossing 



distinct species. And as we must look at the curious 



and complex laws governing the facility with which 



trees can be grafted on each other as incidental on 



unknown differences in their vegetative systems, so I 



believe that the still more complex laws governing the 

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